Many personal, commercial, and military aircraft are powered by gas turbine engines. Gas turbine engines are also used to provide ground-based power for many applications that include ground vehicle power, electrical power generation, and pumping of various liquids and gases. All these gas turbines employ compressors of two main types, that is, (1) axial compressors and (2) centrifugal compressors. Centrifugal compressors turn airflow perpendicular to the axis of rotation. Centrifugal compressors do work on the airflow by accelerating the flow to the higher blade velocity that occurs at the outer diameter of each blade.
Referring now to FIGS. 1A and 1B, a single stage centrifugal compressor and a multi-stage centrifugal compressor are shown. Generally, centrifugal compressor performance is characterized by the pressure ratio across the compressor, the rate at which air or other gases are pumped through the compressor, and an efficiency factor that indicates how much additional work is required relative to an ideal compressor.
Referring now to FIG. 2, a gas turbine engine 10 equipped with a single stage centrifugal compressor 12, without any axial compressor stages on its shaft, that is turned by a single stage turbine 14 is shown. During operation, both the static structures and rotating structures of the centrifugal compressor 12 move. The relative motions of the static structures alone cause the static wall to move away and toward the rotating blades. In addition, the deflections of the structures caused by pressure, radial and axial thermal expansion, and radial and axial tolerance stack-ups lead to open compressor clearances. The pressure deflections in absolute terms relative to a reference point within the compressor tend to be at their maximum at take-off power or a maximum power condition in flight. However, absolute maximum deflection alone does not set the contour of the static structure adjacent to the rotor. The contour is set by another significant mismatch between the contour of the static wall and the rotating blades which arises from the different rates of thermal expansion of the rotor and the static wall parts. The worst-case scenario that exaggerates the effect of the thermal mismatch is when a “hot-reburst” occurs. A hot-reburst is a sequence where high power is set for a long time, followed by a critical low-power period-and then an acceleration back-to-high-power is performed. This circumstance will set the contours of the rotating and static structure and the magnitude of the mismatch relative to steady state mid-power such that at mid-power there is a significant mismatch between the two walls.
FIG. 3 illustrates a cold build clearance 16 between a static wall contour 18 and a rotor blade 20 of the centrifugal compressor 12. When the turbine engine is not operating, the static wall contour 18 and rotating blade contour 20 do not match. These static and rotating contours 18, 20 and the cold build clearance 16 between them are configured so that the static and rotating contours (18 and 20) will coincide (within some margin to accommodate manufacturing variations and prevent rub) at the most severe transient “pinch point” condition. Thus, a perfect matching of the contours of the rotor blades and the static structures 18, 20 for cruise or a mid-power condition is not possible because the hot re-burst transient condition sets the contour and the minimum clearance between the static structure and rotating structure. FIG. 4 shows the static structure contour 18 with respect to the rotating structure contour 20 at a transient minimum state (“hot re-burst”) 22, a steady state (“cruise”) 24, and a transient maximum state (“decel”) 26. The dotted lines of the hot re-burst 22 and decel 26 indicate a shifting static structure moving forward and aft, inward and outward, all at different rates than the rotating structure contour which is a fixed reference for purposes of explanation. It is important to note that the cruise contour 24 falls within the hot re-burst 22 and decel 26 and therefore can never be perfectly matched to the rotor.
Therefore, there exists a need to compensate for the relative motion of both the static structures and rotating structures and maintain a minimum clearance between the static structures and rotating structures during steady state and transient operation.